Special characters?

Hi,
How can I create the following characters: ïäéèçë (and others) in an
X-session without copy pasting them from Gnome Character map? Thus I
want to use the keyboard to use these characters in an easy way. Also,
I'd like to bind keys to them myself. I have seen the deathkeys option
in the gnome keyboard configuration, but that basically binds "e to ë,
but I don't seem to be able to configure it myself.
I have seen Xmodmap, but configuring that also takes a lot of time.
Any other options?
Regards,
Ron